A media presentation forms a collection of encoded and deliverable versions of media content that is usually composed of one or several media content components also called media items such as audio, video or text. A combination of media items or components could also be considered. They can be sent from a server to a client for being jointly played by the client device. Media content is downloaded by the client from a server.
In this context, a new standard called DASH (for “Dynamic Adaptive Streaming over HTTP”) has recently emerged (see “ISO/IEC 23009-1, Dynamic adaptive streaming over HTTP (DASH), Part 1: Media presentation description and segment formats”). This standard enables to create an association between a compact description of the content(s) of a media presentation and the HTTP addresses. Usually, this association is described in a file called a manifest file or description file. In the context of DASH, this manifest file is a file also called the MPD file (for Media Presentation Description).
When the client device gets the MPD file, the description of each encoded and deliverable version of media content can be easily known by the client. By reading the manifest file, the client is aware of the kind of media content components proposed in the media presentation and is aware of the HTTP addresses for downloading the associated media content components. Therefore, it can decide which media content components to download (via HTTP requests) and to play (decoding and play after reception of the media data segments).
The DASH standard allows describing a whole media presentation into one or more periods of time (noted “Period”). A Period describes of a set of media content components that have a common timeline as well as relationships on how they can be presented. Each of these Periods can contain the same set of media content components or a completely independent set of media content components (e.g. a period of time corresponding to a movie followed by another period of time corresponding to an advertisement followed itself by another period of time corresponding to the continuation of the movie . . . ).
In addition to this association, the DASH standard proposes, within a Period, to split each media content component into segments representing smaller periods of time. Thus it describes the association between HTTP addresses (or URLs) and the compact description of each media content component over a small period of time. A segment may contain data for more than one media content component if media content components are multiplexed (combination of media items or components).
The invention focuses on a video description in a manifest file (by taking the DASH MPD as an example). Even if the other elements of the media representation (e.g. audio, text . . . ) are not explicitly described, they can easily be incorporated in a more global media description as will be explained below.
More particularly, the invention focuses on media presentations that are composed of multiple video or audio content components. Those multiple video content components may correspond to multiple alternate videos in a multi-view or multi-camera media presentation. It may also correspond to different tiles (i.e. spatial sub-parts) when a high resolution media presentation is split into multiple spatial sub-parts, each spatial sub-part forming a video content component. By splitting a video into tiles, if the user of a low capacity application wants to display or focus on sub-parts of the video, only the tiles corresponding to the sub-part can be transmitted. This process allows keeping a video portion with a good quality.
In the context of DASH, the known standard “ISO BMFF” (“Base Media File Format”) is used to encapsulate media content components into media data segments in order to form the media presentation. For instance, video content components can represent video bit streams encoded with the format MPEG4, AVC, SVC, HEVC, scalable or multi-view HEVC.
Classically, DASH defines identifiers as part of the attributes that defines the different elements that compose an MPD (e.g. AdaptationSet or Representation ‘id’ attributes). Those identifiers are defined only within the scope of a time period called “Period” for DASH and so they don't allow to follow the same content across Periods. DASH defines a specific identifier called “AssetIdentifier” at Period level that allows recognizing that a Period is the temporal continuity of one or more previous Periods, but it doesn't always allow recognizing the same media content component over different periods in all cases.
Moreover identifiers can be provided at a finer level called Adaptation Set level. The adaptation set level corresponds to the parameters related to a media item or combination of media items for the given time period. The document m33112 (Yasuaki Tokumo and al., “DASH: signaling the continuity of adaptation sets across periods”, 108 MPEG meeting; Valencia; (MOTION PICTURE EXPERT GROUP or ISO/IEC JTC1/SC29/WG11), m33112, April 2014) describes an identifier inserted at the Adaptation Set level.
Nevertheless, the existing solutions does not allow having an efficient parsing of the MPD by the user in many situations, in particular when the media content is partitioned, for example spatially partitioned.
Indeed even if solutions exist for following media content and its parameters over several periods, these solutions do not allow:                following a specific object defined over a portion of the media content, especially when the object does not belong to the same portion of media content for different periods;        indicating that an adaptation set should be related to several different media contents or indicating that several spatial sub-parts are the temporal continuity in space across different time periods.        
Moreover even if solutions exist for following media components over different periods, it may be needed to identify for a given period a media component having common contents with another media component.